1. Field of the Invention
The present invention relates to an optical information recording medium and, more particularly, to an optical information recording medium capable of recording and reproducing information by laser beam irradiation. Particularly, it relates to a heat mode type optical information recording medium suitable for recording information by having a laser beam with a short wavelength of 380 to 500 nm irradiated thereon.
2. Description of the Related Art
Optical information recording media (optical disks) capable of recording information only once with a laser beam are conventionally known. Such optical disks are also referred to as write-once CDs (CD-Rs) and in a typical structure thereof, a recording layer comprising an organic compound such as an organic dye, a light reflective layer comprising a metal such as gold, and a protective layer made of a resin are laminated successively, in this order, on a transparent disk-shaped substrate. Information is recorded to a CD-R by irradiating a near-infrared laser beam (usually a laser beam with a wavelength near 780 nm) thereon, in which the irradiated area of the recording layer absorbs the beam. The temperature of the irradiated area increases, causing the optical characteristics of the area to undergo physical or chemical changes (e.g., the formation of pits) and the information is thus recorded.
With regards to reading (reproduction) of information, this is also conducted by irradiating a laser beam with a wavelength identical to that of the recording laser beam. Information reproduction from the CD-R is conducted by detecting the difference of the reflectivity in the recording area between the areas where the optical characteristics have been changed (recorded area) and not changed (unrecorded area).
In recent years, there has been a demand for optical information recording media possessing higher recording density. To meet this demand for greater recording capacity, an optical disk referred to as a write-once digital versatile disk (DVD-R) has been proposed (for example, see Nikkei New Media special volume xe2x80x9cDVDxe2x80x9d, published in 1995). The DVD-R is configured by appending two disks, each usually formed by laminating a recording layer containing an organic dye, a light reflection layer and a protective layer, in this order, on a transparent disk-shaped substrate in which guide grooves (pre-grooves) for laser beam tracking are formed. The pre-grooves occupy a narrow area of the CD-R, specifically one-half or less of the CD-R (0.74-0.8 xcexcm) and the recording layers of the disk are formed towards the inner portion of the disk. The CD-R can also be configured so that a disk-shaped protective substrate is included with the recording layer formed towards the inner portion of the disk. Information is recorded to and reproduced from the DVD-R by irradiating a visible laser beam thereon (usually a laser beam with a wavelength of about 630 nm to 680 nm), and thus, recording at a density higher than that of a CD-R is possible.
However, considering factors such as the recent spread of networks (e.g., Internet), the rapidly increasing popularity of high-vision TV, and the emergence of high definition television (HDTV) broadcasting, cheap and convenient recording media that are capable of recording image information at even larger capacity are required. While DVD-Rs sufficiently serve as high-capacity recording media at present, demand for larger capacity and higher density has increased. Hence, development of recording media capable of meeting such requirements has become essential, and much effort is being made to develop recording media with larger capacity capable of recording at high density at a shorter light wavelength than that of DVD-Rs.
Information recording and reproduction methods using an irradiated laser beam at a wavelength of 530 nm or less, wherein the laser beam is irradiated from the side of the recording layer to the side of the reflection layer in optical information recording media having organic dye recording layers, have been proposed. Examples of such media include JP-A Nos. 4-74690, 7-304256, 7-304257, 8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-113504, 2000-149320, 2000-158818, and 2000-228028. In the methods described above, information is recorded and reproduced by irradiating a blue laser beam (wavelength: 430 nm, 488 nm) or blue-green laser beam (wavelength: 515 nm) onto an optical disk having a recording layer containing porphyrine compounds, azo dye, metal azo dye, quinophthalone dye, trimethincyanine dye, dicyanovinylphenyl skeleton dye, cumarin compounds and naphthalocyanine compounds.
Further, when considering compatibility with CD-R systems presently in use, optical information recording media capable of recording and reproducing with laser beams of two different wavelength regions have also been proposed. JP-A Nos. 2000-141900, 2000-158816, 2000-185471, 2000-289342 and 2000-309165 propose optical information recording media capable of recording and reproducing by any laser beams in the near-infrared region of about 780 nm and visible laser beams in the region of about 650 nm. These technologies propose using CD-R dyes and DVD-R dyes in admixture.
According to studies conducted by the present inventors, the optical disks described in the above patent publications can not obtain the sensitivity required for practical use when recording information by irradiation of a short wavelength laser beam at a wavelength of 380 to 500 nm. Furthermore, other recording characteristics such as reflectivity and modulation degree are not at satisfactory levels. Clearly, these technologies require further improvement. Particularly, in the optical disks described in the above patent publications, the recording characteristics actually deteriorated when irradiating a laser beam with a wavelength of 380 to 500 nm.
The present invention has been accomplished in view of the foregoing existing problems and intends to provide an optical information recording medium possessing superior recording characteristics, capable of high-density recording and reproduction of information by irradiation of a short wavelength laser beam, at a wavelength of 380 to 500 nm and, more particularly, a laser beam at a wavelength of near 405 nm, which has wide general usability.
In order to achieve the aforementioned purpose, the optical information recording medium according to the present invention provides a medium having, on a substrate, a recording layer capable of recording information by irradiation of a laser beam at a wavelength of 380 to 500 nm, in which the recording layer contains a compound represented by the following general formula (I) (hereinafter occasionally referred to as xe2x80x9ccompound of the inventionxe2x80x9d). The compound of the invention has an absorption band of approximately 340 nm. When the compound of the invention is used as a recording material for the recording layer, an optical information recording medium having satisfactory recording and reproduction characteristics, exhibiting high sensitivity even to a short wavelength laser beam at a wavelength of 380-500 nm, and providing high reflectivity and a high degree of modulation can be obtained. 
where R1 to R6 each represents, independently, a hydrogen atom or a substituent, and M represents a metal atom and n represents an integer of 1 or greater.
In the general formula (I) described above, the metal representing M is preferably copper, nickel, iron, cobalt, palladium, magnesium, aluminum, zinc, silicon or rare earth elements. Further, each of the substituent R1 to R6 is preferably an alkyl group of 1 to 20 carbon atoms, an aryl group of 6 to 14 carbon atoms, an aralkyl group of 7 to 15 carbon atoms, a heterocyclic ring group of 1 to 10 carbon atoms, an alkoxy group of 1 to 20 carbon atoms, an aryloxy group of 6 to 14 carbon atoms, an acyl group of 2 to 21 carbon atoms, an alkyl sulfonyl group of 1 to 20 carbon atoms, an aryl sulfonyl group of 6 to 14 carbon atoms, a carbamoyl group of 1 to 25 carbon atoms, a sulfamoyl group of 0 to 32 carbon atoms, an alkoxy carbonyl group of 1 to 20 carbon atoms, an aryloxy carbonyl group of 7 to 15 carbon atoms, an acylamino group of 2 to 21 carbon atoms, an amino group, a sulfonyl alkylamino group of 1 to 20 carbon atoms, a sulfonyl arylamino group of 6 to 14 carbon atoms, a sulfonyl amino group, or a halogen atom. Further, n is preferably an integer of 1 to 4.
The optical information recording medium according to the invention preferably has further a light reflection layer made of metal between the substrate and the recording layer and, more preferably, it has further a cover layer on the recording layer for protecting the recording layer. Further, it is preferred that pre-grooves at a track pitch of 0.2 to 0.5 xcexcm are formed on the surface of the substrate on the side provided with the recording layer.